It's the pent-up emotion / tension from the anger. Anger produces adrenaline and stress, and once it dies down, often that adrenaline and stress is still there so crying is an excellent way to get it out of your system in a more peaceful way / release.

Also, the reason you are angry in the first place is most likely because you feel hurt, upset, sad, frustrated, unloved, rejected, threatened (your happiness is threatened), disrespected, and so on...all of these feelings, if you skipped the angry part, would result in crying as the basic expression of those feelings. Anger is often a complication of sadness and hurt. It's easier to get angry than to simply go straight to expressing how you really feel - which is tears.